System and method for laser-based, non-evaporative repair of damage sites in the surfaces of fused silica optics

ABSTRACT

The present disclosure relates to a system for repairing a damage site on a surface of an optical material. The system may have an Infrared (IR) laser which generates a laser beam having a predetermined wavelength, with a predetermined beam power, and such that the laser beam is focused to a predetermined full width (“F/W”)  1 /e 2  diameter spot on the damage site. The IR laser may be controlled to maintain the focused IR laser beam on the damage site for a predetermined exposure period corresponding to a predetermined acceptable level of downstream intensification. The laser beam may heat the damage site to a predetermined peak temperature which causes melting and reflowing of material at the damage site to create a mitigated site.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/707,053 filed on Dec. 6, 2012 (now allowed), which claims the benefitof U.S. Provisional Application No. 61/567,581, filed on Dec. 6, 2011.The entire disclosures of each of the above applications areincorporated herein by reference.

STATEMENT OF GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

FIELD

The present disclosure relates broadly to systems and methods forrepairing damage sites in the surfaces of fused silica optics, and moreparticularly to a laser-based system and method which is well suited forthe non-evaporative repair of small damage sites in surfaces of fusedsilica optics.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

High energy pulses of UV light can be especially problematic forcreating damage in the surface of fused-silica optics. Even moreconcerning, once created, damage sites can increase in size (grow) uponexposure to subsequent UV pulses and render the optic useless. For highpower laser applications, such as used for fusion energy researchconducted in the National Ignition Facility at Lawrence LivermoreNational Laboratory, the fused-silica optics are expensive and thereforea long service life is very desirable. Surface damage sites in theseoptics will typically initiate with a diameter in the range of tens ofmicrons and then will grow exponentially in size upon further UVexposure. The size at which a damage site will render an optic uselessdepends on the specific application, however, a nominal maximum size isabout 1 mm. For the present disclosure, the interest is in treatingsurface damage sites with diameters that are typically less than about110 um.

Because of its very strong absorption in silicate glasses, carbondioxide (CO₂) lasers operating at 10.6 μm wavelength have been usedsuccessfully to improve the damage threshold or to arrest (mitigate) thegrowth of laser damage in the surface of fused silica optics. CO₂laser-based mitigation can be very effective in repairing the defectsassociated with a damage site by removal of the damaged or defectivematerial from the optic's surface through evaporation (i.e., by heatingto temperatures greater than about 2500K), or by melting and re-flowingthe damaged material at temperatures below the evaporation point (i.e.,temperatures below about 2500K).

Over the past three decades, several CO₂ laser-based mitigationtechniques have been successfully developed for controlling theinitiation and growth of surface damage sites in fused-silica. For amitigation technique (herein referred to as a “mitigation protocol”) tobe effective, it is strongly preferable that the technique satisfiesthree acceptability requirements: 1) prevent the re-initiation and/orgrowth of a damage site upon subsequent exposure to UV laser pulses; 2)leave behind a low enough level of residual stress that nearby features(i.e. flaws and/or cracks) in the surface will not subsequently inducefracture; and 3) have a final physical shape that will not causeunacceptable downstream intensification when a laser beam passes throughthe mitigated site. In particular, there could be features of themitigated site that cause constructive interference that producesintensification sufficient to damage downstream optics, or the exitsurface of the mitigated optic, should the mitigated site be on itsinput surface. Under CO₂ laser-heating conditions and at peaktemperatures above 2200K, thermo-capillary driven flow can produce afinal shape to the mitigated site that can adversely refract anddiffract UV laser light passing through it upon subsequent exposure. Oneof the most common diffracting features in the final shape of themitigated site is a “rim” surrounding the crater produced by themitigation procedure. These rims frequently are responsible forproducing unacceptably high intensification. Furthermore, any surfacestructure in the mitigated site that would approximate a positive lenscould also produce high intensification. It was realized in developingthe subject matter of the present disclosure that meeting requirement 3)was an especially significant challenge in developing a successfulmitigation protocol.

Previous to 2005, prior art systems and methods addressed onlyrequirements 1) and 2) described above. Indeed, Brusasco et al.,“Methods for mitigating surface damage growth in NIF final optics,” L.W., Proceedings of SPIE 4679, 23, in 2002, teach that a singleapplication of a 5 mm, 10.6 um wavelength CO₂ laser beam in the powerrange 17-35 W applied for 1 second was 100% effective at mitigatinggrowth of UV laser-induced surface damage in a fused silica sample.Further, based on this,

Hackel et al. in 2003 patented a “Method for producing damage resistantoptics” (U.S. Pat. No. 6,518,539). The Hackel et al. invention offers ageneral description of a CO₂ mitigation protocol as a mitigation processon a fused-silica optic that is performed with a CO₂ laser to locallysoften the material within, and in the immediate vicinity of, eachdamage site, to thus anneal out each damage site. The Hackel et al.patent describes using a CO₂ laser having its power density and durationcontrolled to thermally soften fused-silica in a way that minimizesvaporization of material and thermally induced stress in the material.However, Hackel et al. makes no mention or suggestion of controllingdownstream intensification by controlling the final surface shape of theCO₂ laser-mitigated site.

It was not until 2006 that researchers began to appreciate the uniquedifficulty presented by the downstream intensification produced by thefinal surface shape of the mitigated damage site. For instance, withBass et al. in 2006, the mitigation approach was to remove the entiredamage site with several high temperature exposures. This procedureinvolved three scans of a 200 μm diameter laser spot in an inwardlymoving spiral over the damage site. In order to remove (ablate) enoughmaterial to completely mitigate these sites, large areas of the surfacewere heated to temperatures between about 2500K and 3000K for the 150 msduration of the scan. This resulted in re-deposition of some of theablated material from the site and thermo-capillary driven flow ofsilica along the surface of the mitigation pit. However, thisre-deposited material was prone to further laser damage. As a result,techniques were developed to re-melt this re-deposited material at lowertemperatures. This was done by using the CO₂ laser operated at lowerpower to eliminate the re-deposited material's potential for damage.However, the thermo-capillary driven flow caused the formation of a“bump” at the bottom of the mitigation pit that would very often produceproblematic downstream intensification. It was then found that a secondpass with a lower power CO₂ beam would smooth this “bump” and in certaincases, but not all, alleviate the problematic downstreamintensification.

Guss et al., “Mitigation of growth of laser initiated surface damage infused silica using a 4.6 um wavelength laser,” Proceedings of SPIE 6403,64030M-1, 2007, investigated using a frequency-doubled CO₂ laseroperating at a wavelength of 4.6 um to mitigate damage sites with deepcracks. The motivation for using 4.6 um light was a >25 times longerabsorption length in fused-silica at room temperature compared to thatat 10.6 um. Guss et al. showed that it was possible to mitigate damagesites and subsurface cracks using 4.6 um wavelength light withoutsignificant ablation of the material. The resulting depths of themitigated sites were much shallower than those produced by the methodused by Bass et al. in 2005, and had no “bump” at the pit's bottom. Thislack of a “bump” was due to the reflow of the site being dominated bysurface tension vs. thermo-capillary flow forces. However, in somefraction of the mitigated sites it was observed that a rim was createdthat produced unacceptably high downstream intensification.

In 2008, Matthews et al., “Downstream intensification effects associatedwith CO₂ laser mitigation of fused silica,” Proceedings of SPIE 6720,67200A-1, specifically studied the downstream intensification effectassociated with four particular CO₂ laser mitigation protocols. Theseprotocols were ablative (>2500K) in nature and left mitigation pits withdiameters on the order of 500 um. In all four cases, a rim resultedaround the pit with heights on the order of about 3 um. In one case asecondary application of the CO₂ laser was used, only along the rim, tomodulate (“dimple”) the rim and thus reduce its symmetry. Matthews etal. found that the downstream intensification pattern due to the pitsgenerally had two main components - an on-axis “hotspot” and an off-axis“ring caustic”. Both the “hotspot” and “ring caustic” werequantitatively predicted and experimentally shown to have high enoughintensification to initiate downstream damage for relevant fluences (−8J/cm²) and downstream distances (<150 mm). For the “dimpled” rim case,however, the predicted number of damage sites was essentially zero andno damage was experimentally observed. This implies that the rim was themain culprit driving the intensification and that the resultingintensification could be brought to non-damaging levels by modifying or,in this case, “dimpling” the rim.

SUMMARY

In one aspect the present disclosure relates to a system for repairing adamage site on a surface of an optical material. The system may comprisean Infrared (IR) laser which generates a laser beam having apredetermined wavelength, with a predetermined beam power, and such thatthe laser beam is focused to a predetermined full width (“F/W”) 1/e²diameter spot on the damage site. The IR laser may be controlled tomaintain the focused IR laser beam on the damage site for apredetermined exposure period corresponding to a predeterminedacceptable level of downstream intensification. The laser beam mayfurther be used to heat the damage site to a predetermined peaktemperature which results in melting and reflowing of material at thedamage site of the optical material to create a mitigated site.

In another aspect the present disclosure relates to a system forrepairing a damage site on a surface of an optical material. The systemmay comprise an infrared (IR) laser adapted to generate a laser beamhaving a predetermined wavelength, with a predetermined beam power, andsuch that the laser beam is focused to a predetermined full width(“F/W”) 1/e² diameter spot on the damage site. A computer may beincluded which is configured to control the IR laser. The computer maybe further configured to control the IR laser to maintain the focused IRlaser beam on the damage site for a predetermined exposure periodcorresponding to a predetermined acceptable level of downstreamintensification, to enable the laser beam to heat the damage site to apredetermined peak temperature, which results in melting and reflowingof material at the damage site of the optical material to create amitigated site. The computer may further be configured to control the IRlaser to apply a first, predetermined, continuous level of beam powerfor a first time duration, and a reduced, second beam power level over asecond time duration.

In still another aspect the present disclosure relates to a system forrepairing a damage site on a surface of an optical material. The systemmay comprise an Infrared (IR) laser generating a laser beam having apredetermined wavelength, with a predetermined beam power, and such thatthe laser beam is focused to a predetermined full width (“F/W”) 1/e²diameter spot on the damage site. A focusing lens may be incorporatedfor focusing the laser beam to the 1/e2 diameter spot. At least onemirror may be incorporated for receiving the laser beam and directingthe laser beam toward the focusing lens. A computer may be included forcontrolling the IR laser. The computer may be configured to control theIR laser to maintain the focused IR laser beam on the damage site for apredetermined exposure period corresponding to a predeterminedacceptable level of downstream intensification. The laser beam is usedto heat the damage site to a predetermined peak temperature, whichresults in melting and reflowing of material at the damage site of theoptical material to create a mitigated site. The computer may further beconfigured to control the IR laser to apply a first, predetermined,continuous level of beam power for a first time duration, and a reduced,second beam power level over a second time duration.

In one aspect the present disclosure relates to a method for repairing adamage site on a surface of an optical material. The method may involvefocusing an Infrared (IR) laser beam having a predetermined wavelength,with a predetermined beam power, to a predetermined full width (“F/W”)1/e² diameter spot on the damage site. The focused IR laser beam may bemaintained on the damage site for a predetermined exposure periodcorresponding to a predetermined acceptable level of downstreamintensification. The focused IR laser beam heats the damage site to apredetermined peak temperature, which melts and reflows material at thedamage site of the optical material to create a mitigated site.

In another aspect a method of repairing a damage site on a surface of anoptical material is disclosed which may involve focusing an Infrared(IR) laser beam of a predetermined wavelength, with a predetermined beampower, to a predetermined full width (“F/W”) 1/e² diameter spot on thedamage site. The focused IR laser beam may be maintained on the damagesite for a predetermined exposure period. The focused IR laser beam maybe used to heat the damage site to a predetermined peak temperature ofbetween about 1900K to about 2500K. This causes melting and reflowing ofthe optical material at the damage site to create a mitigated site.

In still another aspect the present disclosure involves a system forrepairing a damage site on a surface of an optical material. The systemmay comprise an Infrared (IR) laser generating a laser beam having apredetermined wavelength, with a predetermined beam power, and such thatthe laser beam is focused to a predetermined full width (“F/W”) 1/e²diameter spot on the damage site. The IR laser may be configured tomaintain the focused IR laser beam on the damage site for apredetermined exposure period corresponding to a predeterminedacceptable level of downstream intensification. The focused IR laserbeam may be used to heat the damage site to a predetermined peaktemperature, which results in melting and reflowing of material at thedamage site of the optical material to create a mitigated site.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way. Inthe following drawing figures:

FIG. 1 is a high level block diagram illustrating one implementation ofa system for performing damage site mitigation on a material such as afused-silica optic;

FIG. 2 is a graph of a laser beam power profile generated by the laserof FIG. 1 that makes use of a “truncated”, linearly decreasing rampportion;

FIG. 3 is a graph showing an alternative laser power profile thatincludes a linearly decreasing-to-zero portion;

FIG. 4 is a graph of another alternative laser power profile thatincorporates a reduced power “step” portion;

FIG. 5 is a graph illustrating a plot of estimated residual stress (MPa)versus CO2 laser power applied for the second time duration (t2)portions of different laser power profiles; and

FIG. 6 is chart illustrating the results of damage tests of mitigatedsites in the surface of fused-silica optics, conducted using alarge-aperture (about 3 cm) laser beam, where the damage sites have beenmitigated in accordance with a methodology of the present disclosure.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

Referring to FIG. 1, one embodiment of a system 10 is shown for carryingout one preferred methodology of the present disclosure. In this examplea 20W, quasi-continuous-wave 10.6 um wavelength CO₂ laser 12 is usedthat outputs a laser beam 14 preferably having a Gaussian spatialprofile. The laser beam 14 may be used at preferably about 10 kHz and atan approximate 50% duty cycle. The laser's beam 14 is allowed tofree-propagate, using ZnSe mirrors 16 and 18, to a 7.5 cm focal lengthaspheric ZnSe lens 20. The lens 20 focuses the laser beam 14 to anapproximate 2 mm diameter (“FW” 1/e²) spot on the surface of a material22. In this example the material 22 is a fused-silica optic (referred tohereinafter as “fused-silica optic 22”) having a damage site 22 a of nomore than about 110 um in diameter. The laser's 12 power as a functionof time may be controlled via a computer 24, for example a personalcomputer or laptop, programmed to control the laser 12 to cause thelaser to generate a beam with a predetermined power profile.

Referring to FIG. 2, a graph is shown of one specific predeterminedpower profile, which is indicated by reference numeral 26, and whichwill be discussed in greater detail in the following paragraphs. Thegraph of FIG. 2 represents the average CO₂ laser 12 power versusexposure time of the beam 14 to the fused-silica optic 22.

The approach to developing the subject matter of the present disclosurehas focused in part on identifying a set of process parameters that,when applied to damage sites in the surface of the fused-silica optic22, would not cause the evaporation of any material, and stillsimultaneously meet all three of the acceptability requirementsdiscussed above (i.e., prevent the re-initiation and/or growth of adamage site upon subsequent exposure of the fused-silica optic 22 toultraviolet (UV) laser pulses; control residual stress so that nearbyflaws or cracks in the surface of the fused-silica optic 22 will notsubsequently induce fracture; and produce a mitigated site in thefused-silica optic 22 having a final physical surface shape which doesnot produce unacceptable downstream intensification when a laser beampasses through the mitigated site).

For a given beam 14 diameter and power, it was observed that theeffectiveness of a protocol against re-initiation/growth increased withexposure time. Additionally it was observed that as the exposure time ofthe CO₂ laser beam 14 to the fused-silica optic 22 increased, thedownstream intensification also increased. So for the presentdisclosure, a significant challenge was presented in discovering anoptimum balance between power (for a fixed beam diameter, i.e. peaktemperature) and exposure time where a damage site is reflowed to thepoint of being adequately mitigated (100% effective), but not modifiedto the point where downstream intensification becomes a potentialproblem.

Further, in the development of the subject matter of the system andmethodology of the present disclosure, it was determined that rimformation around the mitigated damage site was not the specific featureof the final shape of the mitigated damage site on the fused-silicaoptic 22 that was principally responsible for causing unacceptably highdownstream intensification. Instead, it was discovered that theresulting shape of the walls near the top of a crater formed at themitigated damage site are most responsible for producing downstreamintensification. It was also observed that with prior art systems andmethods for site mitigation, the reflowed walls diffract and focus thelight into a “hot” caustic or ring. The caustic or ring can easily havedownstream intensifications of about 3:1.

The present disclosure therefore recognizes the need for highlyspecific, controlled ranges for a plurality of process specificparameters to achieve successful damage site mitigation in thefused-silica optic 22. Most specifically, the present disclosureidentifies and recognizes the need to closely control at least fourimportant parameters: 1) beam diameter (“FW” 1/e²); 2) beam power; 3)exposure time; and 4) peak temperature of the damage site. It has beendiscovered that it is strongly preferable that the CO₂ laser beam 14 hasa full width (i.e., “FW” 1/e² diameter) sufficiently large to bring theentire damage site to within a closely specified temperature range for aclosely specified exposure time. In particular, it has been discoveredthat the entire damage site should be treated with a beam that producesan irradiance (W/cm²) that results in preferred temperatures betweenabout 1900K and about 2200K at the damage site of the fused-silica optic22. This preferred temperature range should be maintained long enough tomitigate the laser damage, but not so long that the morphology of thedamage site changes to the point where the downstream intensificationbecomes problematic.

In view of the above, one highly important aspect of the presentdisclosure has been developing a methodology to find a satisfactorydwell time for a given laser irradiance. With this methodology, aplurality of operations may involve: 1) creating a set of damage siteswith a laser; 2) exposing the damage sites to single applications of theCO₂ laser beam at an irradiance to reach the specified temperature rangefor different dwell times; 3) measuring the downstream intensificationfrom the treated sites with a suitable device as a function of dwelltime; 4) selecting the dwell time and irradiance to ensure acceptabledownstream intensification; 5) laser damage testing the damage siteswhich passed the intensification test to a desired fluence to ensure thedamage threshold is acceptable; and 6) setting the exposure conditionsto satisfy the damage threshold and intensification metrics as describedabove.

In view of the above, the diameter (“FW” 1/e²) of the laser beam 14preferably should be maintained in the range of about 1.9 mm to about2.1 mm. For this beam diameter, the laser beam 14 power should bemaintained in the range of about 9.75W to about 10.25W to achieve thedesired irradiance levels and temperature. The time that the damage siteis exposed to the CO₂ laser beam 14 preferably is kept within the rangeof about 210 seconds to about 270 seconds. The power profile 26 in FIG.2 illustrates this duration as being about 264 seconds. If this andother ones of the parameters described above are not controlled towithin the specified ranges, the result could potentially be a failureto meet one or more of the acceptability requirements.

It has also been discovered that ramping down the CO₂ laser 12 power tojust below the glass transition temperature (—1400K) of the fused-silicaoptic 22 is highly effective in reducing the residual stress left in thefused-silica optic 22 in and around the damage site to an acceptablelevel. Furthermore, it has been found that introducing various “ramps”in the CO₂ laser 12 power are all effective in reducing the residualstress in the material being mitigated. For example, the laser beampower profile 26 shown in

FIG. 2 makes use of a first continuous power level 26 a (P1, 10W) thatis maintained for a first time duration (t1), after which a linearlydecreasing power level 26 b (P2) forming a “truncated” ramp is usedduring a second time duration (t2) followed by an immediate turn off.FIG. 3 shows an alternative laser beam power profile 30 that makes useof a first continuous power level 30 a (P1) for a first duration (t1),and then a second, linearly decreasing to zero power level 30 b (P2, aramp decreasing to zero) over time duration t2. FIG. 4 shows yet anotheralternative laser beam power profile 32. Power profile 32 may have afirst continuous portion 32 a having a first power (P1) for a first timeduration t1, followed by a “step” down to a reduced, continuous powerlevel 32 b (P2) for a second time duration t2, followed by an immediateturnoff. It has been found that each of the power profiles 26, 30 and 32are effective in reducing the residual stress in the fused silica-optic22. It has also been discovered that for the peak temperatures achievedin the fused-silica optic 22, a ramp down (for a 2 mm beam diameter)from about 10W, producing a temperature about 2000K, to about 7W,producing a temperature of about 1400K, over a time of about 24 seconds(as noted in FIG. 2) suitably minimized the residual stress to a pointwhere the probability to see fracture in flaws or cracks with 300 um orless sized features was acceptably low.

FIG. 5 illustrates the test results of estimated residual stress (MPa)versus laser power applied for the power profile 26, which has atruncated ramp, and power profile 32, which has P2 as a “step”. Thesolid circles encased within squares indicate estimated residual stressfor a simple single power level exposure with no ramp or no stepincluded.

It will therefore be apparent to one skilled in the art that the methodsof the present disclosure can be applied to any optical material whichabsorbs optical energy strongly at a predetermined wavelength, in oneexample a 10.6 um wavelength. It should also be appreciated that thesystem 10 and methodology of the present disclosure does not necessarilyrequire a CO₂ laser operating at 10.6 um. These results can be achievedusing other IR lasers operating at different wavelengths which are alsoabsorbed strongly and which thermally modify the material being treated.

The present disclosure demonstrates that a large number of damage siteswith pre-mitigation diameters of up to about 110 um, and possibly evenhigher, on a fused-silica optic can be successfully mitigated (i.e. meetall three requirements discussed above for successful mitigation).Furthermore, the system 10 and methodology of the present disclosuredoes not cause re-deposited debris in or around the mitigated site ascharacterized by high resolution optical microscopy and damage testing.

FIG. 6 lists the results of damage tests conducted using a damage testlaser with a large-aperture (about 3 cm) beam at a wavelength of 351 nm.The damage test laser further produced a flat-in-time temporal pulseshape having a 5 ns period. This damage test laser was used to determinethe damage threshold for mitigated sites prepared in accordance with themethodology of the present disclosure. The damage tests involved using afused-silica sample with mitigated damage sites on one surface. Thesample was mounted in a chamber held at vacuum (10⁻⁵ Torr), andsubjected to single 351 nm pulses at a rate of about one per hour. Thesample was further orientated with the mitigated sites on the outputsurface with the pattern of mitigated damage sites centered in thedamage test laser's beam. FIG. 6 shows that the damage threshold wasgreater than 12 J/cm² for a 5 ns flat-in-time pulse at 351 nmwavelength. In addition, the shapes of the mitigated damage sitesresulted in measured downstream intensifications that were acceptable,that is, below that at which downstream optics >10 mm away would bedamaged (i.e. intensification less than about 2:1). Finally, it will benoted that the residual stress left in the fused-silica optic 22 in andaround the final mitigated site would not cause additional fracture orcrack growth for nearby surface flaws or cracks with features as largeas 300 um.

It will be appreciated then that the system 10 and methodology of thepresent disclosure provides a means to repair damage sites in thesurface of a material, which in one example has been given as afused-silica optic. It will be appreciated that the system 10 andmethodology of the present disclosure may be applied to other materials,and especially materials suitable for forming optics (any glass system,glass systems doped with absorbing ions (i.e. filter glasses), andcoated optical materials). Importantly, with the system 10 andmethodology of the present disclosure a number of important parametersare met, namely: 1) preventing the re-initiation and/or growth of adamage site upon subsequent exposure to UV laser pulses; 2) leavingbehind a low enough level of residual stress so that nearby features(i.e. flaws and/or cracks) in the surface of the material will notsubsequently induce fracture; and 3) producing a mitigated site whichhas a final physical surface shape that will not cause unacceptabledownstream intensification when a laser beam passes through themitigated site.

While various embodiments have been described, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the present disclosure. The examples illustrate variousembodiments and are not intended to limit the present disclosure.Therefore, the description and claims should be interpreted liberallywith only such limitation as is necessary in view of the pertinent priorart.

What is claimed is:
 1. A system for repairing a damage site on a surfaceof an optical material comprising: an Infrared (IR) laser generating alaser beam having a predetermined wavelength, with a predetermined beampower, and such that the laser beam is focused to a predetermined fullwidth (“F/W”) 1/e² diameter spot on the damage site; the IR laser beingcontrolled to maintain the focused IR laser beam on the damage site fora predetermined exposure period corresponding to a predeterminedacceptable level of downstream intensification; and the laser beam beingused to heat the damage site to a predetermined peak temperature, whichresults in melting and reflowing of material at the damage site of theoptical material to create a mitigated site.
 2. The system of claim 1,further comprising a computer for controlling the IR laser.
 3. Thesystem of claim 2, wherein the computer controls the IR laser toprovide: a first, predetermined, continuous level of beam power for afirst time duration; and a reduced, second beam power level over asecond time duration.
 4. The system of claim 2, wherein the computer isconfigured to control the IR laser such that material at the damage siteof the optical material is melted and reflowed without causingevaporation of the material.
 5. The system of claim 2, wherein thecomputer is configured to control the IR laser to provide: a first,predetermined, continuous level of beam power for a first time duration;and wherein the computer controls the IR laser so that the beam power isramped down linearly to zero over a second time duration.
 6. The systemof claim 2, wherein the computer is configured to control the IR laserto provide: a first, predetermined, continuous level of beam power for afirst time duration; and to subsequently reduce the beam power from theIR laser to a second, predetermined continuous level, in step fashion,for a second time duration.
 7. The system of claim 2, wherein thecomputer is configured to control the IR laser beam power so that atemperature of the optical material is reduced from the predeterminedpeak temperature to below a softening temperature of the opticalmaterial, to reduce stress in the optical material.
 8. The system ofclaim 2, wherein computer is configured to control the IR laser so thatthe predetermined exposure period comprises a length of time betweenabout 210 seconds to about 270 seconds.
 9. The system of claim 2,wherein the computer is configured to control the laser beam power towithin a range of about 9.75 watts to about 10.25 watts to achieve adesired irradiance level and the predetermined peak temperature.
 10. Thesystem of claim 2, wherein the computer is configured to control the IRlaser so that the laser beam is modulated at a frequency of about 10 kHzwith an approximate 50% duty cycle.
 11. The system of claim 1, furthercomprising an aspherical lens disposed to receive the laser beam and tofocus the laser beam to a 1/e² spot size.
 12. The system of claim 1,further comprising at least one mirror for directing the laser beamtoward the damage site on the optical material.
 13. The system of claim1, wherein the IR laser comprises a CO₂ laser.
 14. A system forrepairing a damage site on a surface of an optical material comprising:an infrared (IR) laser adapted to generate a laser beam having apredetermined wavelength, with a predetermined beam power, and such thatthe laser beam is focused to a predetermined full width (“F/W”) 1/e²diameter spot on the damage site; a computer configured to control theIR laser; the computer further being configured to control the IR laserto maintain the focused IR laser beam on the damage site for apredetermined exposure period corresponding to a predeterminedacceptable level of downstream intensification, and the laser beam beingused to heat the damage site to a predetermined peak temperature, whichresults in melting and reflowing of material at the damage site of theoptical material to create a mitigated site; and the computer furtherbeing configured to control the IR laser to apply a first,predetermined, continuous level of beam power for a first time duration,and a reduced, second beam power level over a second time duration. 15.The system of claim 14, wherein the reduced, second beam power comprisesa beam power which is ramped down linearly to zero over the second timeduration.
 16. The system of claim 14, wherein the controller isconfigured to control the IR laser to reduce the beam power to thesecond, predetermined continuous level, in step fashion.
 17. The systemof claim 14, wherein the computer is configured to control the IR laserbeam power so that a temperature of the optical material is reduced fromthe predetermined peak temperature to below a softening temperature ofthe optical material, to reduce stress in the optical material.
 18. Thesystem of claim 14, wherein computer is configured to control the IRlaser so that the predetermined exposure period comprises a length oftime between about 210 seconds to about 270 seconds.
 19. A system forrepairing a damage site on a surface of an optical material comprising:an Infrared (IR) laser that generates a laser beam having apredetermined wavelength, with a predetermined beam power, and such thatthe laser beam is focused to a predetermined full width (“F/W”) 1/e²diameter spot on the damage site; a focusing lens adapted to focus thelaser beam to the 1/e2 diameter spot; at least one mirror positioned toreceive the laser beam and direct the laser beam toward the focusinglens; and a computer configured to control the IR laser to maintain thefocused IR laser beam on the damage site for a predetermined exposureperiod corresponding to a predetermined acceptable level of downstreamintensification, and the laser beam being used to heat the damage siteto a predetermined peak temperature, which results in melting andreflowing of material at the damage site of the optical material tocreate a mitigated site; and the computer further being configured tocontrol the IR laser to apply a first, predetermined, continuous levelof beam power for a first time duration, and a reduced, second beampower level over a second time duration.
 20. The system of claim 19,wherein the computer is further configured to control the IR laser sothat a temperature of the optical material is reduced from thepredetermined peak temperature to below a softening temperature of theoptical material, to reduce stress in the optical material.